The exhaust gas of diesel motors contains not only carbon monoxide (CO), but also hydrocarbons (HC), nitrogen oxides (NOx) and soot particles (PM) with up to 10% by volume of oxygen. Conventional diesel oxidation catalysts that effectively promote the reaction of carbon monoxide and hydrocarbons with the oxygen contained in the exhaust gas to form harmless carbon dioxide (CO2) are well-suited to reduce the amount of carbon monoxide and hydrocarbons. Lowering the amount of nitrogen oxides contained in the exhaust gas through the reduction thereof to form nitrogen is more difficult since this calls for the presence of a sufficient quantity of a suitable reducing agent. Various methods are known from the state of the art in order to achieve this objective such as, for example, the use of so-called HC-DeNOx catalysts that promote the conversion of the nitrogen oxides contained in the exhaust gas—along with the hydrocarbons likewise contained in the exhaust gas—to form nitrogen, or else the use of so-called nitrogen oxide storage catalysts. The method for reducing nitrogen oxides in diesel engine exhaust gases currently used most often is so-called selective catalytic reduction (SCR). In this method, ammonia and/or a compound that breaks down to form ammonia such as, for example, urea or ammonium carbamate is added to the exhaust gas that is to be purified as the reducing agent from a source that is independent of the engine. On a catalyst that is suitable for this purpose, the so-called SCR catalyst the nitrogen oxides contained in the exhaust gas are reacted with ammonia to form harmless nitrogen in a comproportionation reaction.
Soot particles are formed during the combustion process inside the engine under conditions at which the combustion of the diesel fuel is not complete. These particles typically consist of soot to which long-chain hydrocarbons can adhere (Soluble Organic Fraction—SOF). Moreover, depending on the diesel fuel used, a so-called inorganic-soluble fraction, for example, sulfuric acid fractions formed from water vapor and sulfur oxides stemming from the combustion of sulfurous diesel fuel constituents in the combustion chamber of the cylinder can adhere to the soot. Diesel particulate filters are typically used to remove soot particles from the exhaust gas of diesel engines. These are preferably so-called “closed” filters that contain a wall-flow filter substrate through which the exhaust gas that is to be purified is fed. Such wall-flow filter substrates, shown for example in FIG. 1 by a wall-flow filter (1) of a length L, have inflow channels (4) through which the exhaust gas that is to be purified and that contains soot particles flows into the filter, and which are closed (7) on the outlet side (3) of the filter so as to be gas-tight. The exhaust gas, which flows in direction→reaches the so-called outflow channels (6) through a porous wall (5). The outflow channels are closed (7) on the inlet side (2) so as to be gas-tight. The porous wall between the inflow and outflow channels is configured in such a way that only the gaseous fractions of the exhaust gas pass through the pores in the wall, whereas the soot particles (8) are held back in the inflow channels of the filter. In order to prevent clogging of the filter, the soot particles held back in the filter are reacted at suitable exhaust gas temperatures with oxidizing constituents of the exhaust gas (nitrogen dioxide and/or oxygen) at least proportionally to form gaseous carbon monoxide and/or carbon dioxide.
Compliance with current and future emission legislation requires a marked reduction of all noxious gases (CO, HS, NOx) contained in diesel engine exhaust gas as well as a marked reduction of the soot particles contained in the exhaust gas. Methods for exhaust gas purification which constitute solutions to this problem are already known from the state of the art.
Thus, for instance, European patent EP-B 1 054 722 discloses a system for the selective catalytic reduction (SCR) system for the treatment of combustion exhaust gas containing NOx and solid particulates, comprising the following in combination and in the order indicated below: an oxidation catalyst that is effective in converting at least part of the NO of the NOx into NO2 and in increasing the NO2 content in the exhaust gas; a fine particulate filter; a reservoir for a reducing fluid; injection devices for such a reducing fluid that are located downstream from the fine particulate filter; and an SCR catalyst. European patent EP-B 2 123 345 discloses such an arrangement of the following exhaust gas purification aggregates: diesel oxidation catalyst, diesel particulate filter, and SCR catalyst, whereby, however, a catalytically activated diesel particulate filter is used, and whereby a specific distribution of the oxidatively active noble metal components platinum and palladium over the diesel oxidation catalyst and the diesel particulate filter is disclosed that is supposed to result in an improvement in the efficiency of the overall system described in European patent EP-B 1 054 722. Both systems are fundamentally suitable to effectively reduce carbon monoxide, hydrocarbons, nitrogen oxides and soot particles that are present in the exhaust gas of diesel engines. However, both systems have the drawback that, in order to achieve the purification efficiency required with an eye towards upcoming emission legislation, particularly in the case of large diesel engines, very large exhaust gas purification aggregates have to be used for which a great deal of installation space has to be provided. This is especially the case with an eye towards achieving the requisite purification efficiency for nitrogen oxides that have to be converted on the SCR catalyst located on the outflow side.
Particularly in the sector of construction and agricultural machinery, there is very little installation space available for exhaust gas purification aggregates.